1. Field of the Invention
The present invention relates to a laminated-type evaporator forming refrigerant passages by a laminated structure of metal thin plates, and more particularly, to a laminated-type evaporator having an auxiliary heat exchanger to perform heat exchange between mutual internal refrigerants flowing within refrigerant passages.
2. Description of the Related Art
In Japanese Patent Application Laid-Open No. Hei 5-196321 (corresponding to U.S. Pat. No. 5,245,843), the same applicant has proposed a laminated-type evaporator having an auxiliary evaporator to perform heat exchange between mutual internal refrigerants flowing within refrigerant passages. The device disclosed in the foregoing Japanese Patent Laid-Open provides, in addition to a main heat exchanger performing ordinary heat exchange between refrigerant and air, an auxiliary heat exchanger (refrigerant-refrigerant heat exchanger) facilitating heat exchange between refrigerant of an inlet-side of the evaporator and refrigerant of an outlet-side of the evaporator. This device increases the moisture of refrigerant flowing into an inlet tank of the main heat exchanger.
The purpose of this auxiliary heat exchanger is to increase the moisture of the refrigerant flowing into the inlet tank of the main heat exchanger and to put the refrigerant in the inlet tank in a state of approximately liquid single phase. Therefore, when refrigerant is distributed from the inlet tank to a plurality of tubes, it is distributed uniformly to the respective tubes. Moreover, since the inner surfaces of the respective tubes are covered by the liquid refrigerant, the thermal transmission rate at the tube inner surfaces is improved, which improves the cooling performance of the evaporator.
According to experimental investigation by the inventors, however, in the device disclosed in the foregoing Japanese Application Laid-Open, it was discovered that difficulties such as will be described hereinafter occur when a block joint, provided in the evaporator for connecting the refrigerant piping from the pressure-reducing unit side and refrigerant piping to the compressor side to the foregoing auxiliary heat exchanger, is fixed by brazing.
Refrigerant passages in the main heat exchanger of the foregoing evaporator are formed by aligning two thin metal plates of uneven configuration, and the main heat exchanger is structured by laminating the refrigerant passages in a multiplicity of sets. Fins are provided between the foregoing respective sets to enlarge the thermal-transmission surface area of the air side.
Additionally, by laminating several thin metal plates, the auxiliary heat exchanger forms in alternation on the front and rear of the thin metal plates an inlet-side refrigerant passage to introduce refrigerant from the pressure-reducing unit side of the cooling cycle to the main heat exchanger and an outlet-side refrigerant passage to introduce refrigerant from the main heat exchanger to the compressor side. The inlet-side refrigerant passage and the outlet-side refrigerant passage are structured so as to exchange heat between refrigerant flowing through the inlet-side refrigerant passage and refrigerant flowing through the outlet-side refrigerant passage.
The above-described main heat exchanger, auxiliary heat exchanger, and block joint are put into a furnace, heated to a predetermined temperature, and brazed integrally. Because the auxiliary heat exchanger is structured of several thin metal plates laminated alternatingly as described above, density is high and thermal transmission is poor in comparison with the main heat exchanger. That is to say, it is difficult for heat to be transmitted to the interior of the auxiliary heat exchanger. Consequently, in brazing at the above-mentioned predetermined temperature, air bubbles due to faulty brazing occur at the brazing surface between the auxiliary heat exchanger and the block joint.
When air bubbles occur in this way, air exists in the bubbles. Because the clearance between the auxiliary heat exchanger and the block joint is extremely small, moisture contained in the air remains within the air bubbles without escaping. When this moisture is chilled by the evaporator and forms frost, the resulting volume expansion causes large pressure to be applied to an end plate of the auxiliary heat exchanger, and the end plate may be destroyed thereby.
In this regard, the problem of faulty brazing is solved when the above-mentioned brazing temperature is raised further, but if the brazing temperature is raised excessively, the main heat exchanger (and in particular the fins), which has low density and high thermal transfer in comparison with the auxiliary heat exchanger, begins to melt.
Furthermore, the above-described several problems are solved if the above-mentioned brazing temperature is suppressed to the foregoing predetermined temperature and brazing time is lengthened, but this increases the fabrication steps and cost.